Fix ichEnd offset bug in DoubleParser when input has leading whitespace

[jelinekdev]

Fixes #7181

Fixes the whitespace handling in DoubleParser.Parse when the input contains leading whitespace.

TryParse returns ichEnd relative to the sliced span. The subsequent validation compares it against the original span, which can incorrectly return Result.Extra for valid inputs such as " 1.234 ".

This change adjusts ichEnd back to the original span before validating the remaining characters.

Added unit tests covering:

• parsing with leading and trailing whitespace (double)
• parsing without whitespace
• parsing with trailing invalid characters
• parsing with leading and trailing whitespace (float)

[jelinekdev]

@dotnet-policy-service agree